One variation of multicarrier communications, OFDM (Orthogonal Frequency Division Multiplex) is known to be an effective measure against frequency selective fading in mobile communications.
OFDM increases the symbol length by sorting a temporal data symbol sequence into a number of subcarriers (i.e. carrier waves) having orthogonal frequencies, and reduces the impact of inter-code interference of delay waves by attaching guard intervals.
In recent years, such art has been attracting attention that implements high speed multimedia mobile communication by combining wired network communication using optical fiber (e.g., ATM (Asynchronous Transfer Mode) communication) and wireless communication of high reliability such as OFDM.
In ATM communication, for instance, four service classes (i.e. CRB, VBR, ABR, and UBR) are provided, and for each class the QoS (Quality of Service) is assigned. Upon file transfer, cases might occur where a low cell loss rate of approximately 10−9 is required including the application layer.
Consequently, to implement seamless connection of ATM network that uses high-quality optical fiber and wireless communication path, error control (i.e. QoS control) between wireless intervals becomes necessary.
Some ARQ (Automatic Repeat reQuest) schemes suitable for multicarrier communication (i.e. OFDM) have been proposed as effective methods of error control in frequency selective fading communication path.
Public document 1 (H. Atarashi et al “Partial Frequency ARQ for Multi-Carrier Packet Communication” IEICE TRAN. COMMUN. vol. E78-B August 1995) discloses an art of attaching an error detection code on a per data basis corresponding to separate carriers.
Public document 2 (Japanese Patent Publication No.11-55206) discloses an art of: attaching one error detection code per transmission packet taking into account that assigning an error detection code on a per carrier basis results in reduced transmission rates; upon detecting an error, specifying a carrier having received quality below a predetermined level; estimating that the error occurred with this carrier; and transmitting a retransmission request to the transmitting end using a carrier having received quality above a threshold level.
Using the art disclosed in aforementioned public document 2, it is possible to improve the transmission rate compared to the art disclosed in public document 1.
However, to transmit a retransmission request, it is necessary to check the received quality of each of many carriers (i.e. carrier waves), memorize information about all carriers that do not meet predetermined quality, and include in the transmission request information that specify the carrier corresponding to the data to which retransmission is desired. If the number of carriers used increases, the number of bits in the information increases as well, which then becomes a factor for reduced transmission rates.
In addition, processing for selecting the carriers for use upon transmission of the retransmission request is also necessary.
For instance, in OFDM, the number of carriers used is assumed to be up to several thousands, in view of which the above-described prior arts still have many problems that need to be overcome before practical use in multimedia communication and the like which require high speed.